luci-app-tinyproxy: Loosen constraint on upstream target

[cukiernick]

• This PR is not from my main or master branch 💩, but a separate branch ✅
• Each commit has a valid ✒️ Signed-off-by: <my@email.address> row (via git commit --signoff)
• Each commit and PR title has a valid 📝 <package name>: title first line subject for packages
• Tested on: (ARMv8, OpenWrt 24.10.4, Firefox 144) ✅
• ( Optional ) Closes: luci-app-tinyproxy:valid domain name reported as invalid #7958
• Description: (describe the changes proposed in this PR)

---

Upstream target datatype is too strict, rejecting values starting with a dot, which selects all subdomains of provided domain.

From tinyproxy docs:

The site can be specified in various forms as a hostname, domain name or as an IP range:

name matches host exactly
.name matches any host in domain "name"
. matches any host with no domain (in 'empty' domain)
IP/bits matches network/mask
IP/mask matches network/mask

I looked at available datatypes in modules/luci-compat/luasrc/cbi/datatypes.lua and modules/luci-base/htdocs/luci-static/resources/validation.js and there is no suitable predefined type, so I changed it to string